Active Role of Vapor Clouds around Evaporating Sessile Droplets in Microgravity: Marangoni Jets and Electroconvection

A benchmark microgravity experiment (dubbed "ARLES")is analyzed. It concerns evaporation of several-& mu;L sessile dropletswith a pinned millimetric circular contact line on a flat substrateinto a vast calm (here nitrogen) atmosphere at nearly normal conditions.Hydrofluoroether (HFE-7100) is used as a working liquid whose appreciablevolatility and heavy vapor accentuate the contrast between the micro-and normal gravity. A possibility of switching on a DC electric field(EF) of several kV/mm orthogonallyto the substrate is envisaged. We here focus on the findings intimatelyassociated with the visualization of the vapor cloud by means of interferometryand rationalized by means of extensive simulations. In particular,with different degrees of unexpectedness, we discover and explorea Marangoni jet (without EF) and electroconvection (with EF) in thegas, which would otherwise be masked by buoyancy convection. Usingthe same tools, we examine some malfunctions of the space experiment.